The present disclosure relates generally to controlled release therapeutic materials, and more particularly to such materials formed from particle-containing complex porous materials.
Controlled release of biologically active agents using polymeric materials is a powerful means for a wide variety of therapeutics. Such approaches may be especially advantageous for delivery of therapeutic agents to a localized area to substantially circumvent unwanted systemic side effects, to achieve high local dosages without the use of large quantities of therapeutic agents, and/or to prevent unnecessary destruction or denaturing of the agents in the delivery routes (such as in circulation or the gastrointestinal track).
Though different from drug delivery, tissue engineering/regenerative medicine is another interdisciplinary and multidisciplinary field, which aims to generate biological replacements for damaged tissues, failing organs, and dysfunctional body parts. Still young and growing, the field significantly challenges the current treatment modalities represented by transplantation, surgical reconstruction, and the usage of prostheses, each of which has inherent limitations. In tissue engineering/regenerative medicine, an artificial temporary extracellular matrix (a.k.a. scaffold) plays an important role as a 3D guidance for cell adhesion, proliferation, and new tissue organization. These scaffold materials are compatible with cells and biological host environments. Such scaffolds also possess the appropriate porous structure suitable for cell penetration, nutrient supplies, and metabolic waste removal. Desirably, they may also encourage needed cell-cell and cell-matrix interactions; and may be biodegradable to eventually fade away after fulfilling the purpose as templates (as this may obviate long-term foreign body reactions and associated complications).
In addition to cells and scaffolds, biological signaling may also be necessary for cell function and tissue regeneration. Endogenous signaling molecules are often not sufficient in quantities or types for regeneration purposes, where the addition of signaling molecules is desirable. For example, when stem cells are used for regeneration, the lineage control often relies on the use of differentiation factors. For in vitro cultures, addition of biological molecules in a culture medium either in a continuous or discrete fashion is often possible. In an in vivo situation, such addition of factors to a cell-scaffold construct or regenerating tissue may be very challenging, if not virtually impossible. Due to the sensitivity of cells to the concentration of biological molecules and because of the short half-lives of many of these agents, controlled release technologies are being introduced into the field of tissue engineering and regenerative medicine.
Many controlled release techniques may potentially be introduced into tissue engineering scaffolds. Those that are relatively easy to adapt have been recently utilized for tissue engineering applications. For example, hydrogels have been used as both drug delivery matrices and tissue engineering scaffolds. Further, growth factors, along with cells, have been incorporated into hydrogel scaffolds to enhance regeneration.
Similarly, biological agents have also been directly added into a polymer or a blend of polymers, and the mixture is then used to fabricate a scaffold. For example, ascorbate-2-phosphate and dexamethasone (AsAP and Dex, agents to enhance osteoblastic differentiation of cells) have been mixed in chloroform, which was used to dissolve poly(d,l-lactide-co-glycolide) (PLGA). This resulted in a mixture (Dex and AsAP suspended in a PLGA solution). This polymer suspension was mixed with salt particles to fabricate a scaffold using the salt-leaching technique, i.e., after the evaporation of solvent, the salt particles were leached away by water to form pores. Others used a PLLA-PDGF (platelet derived growth factor) emulsion to fabricate foams via freeze-drying, polymers mixed with plasmid DNA to fabricate meshes via electrospinning, or a PLGA-bFGF (basic fibroblast growth factor) emulsion to fabricate porous materials using supercritical CO2 foaming.
There also have been attempts to modify a scaffold with biologically active agents using certain coating techniques. For example, collagen matrix was partially dipped in growth factor solutions, such as bFGF, HGF (hepatocyte growth factor), PDGF-BB, VEGF (vascular endothelial growth factor), or IGF-1 (insulin like growth factor-1), to coat these factors on the matrix. Porous bioglass materials have been coated with a laminin solution to allow for subsequent slow release. An emulsion of a polymer solution and vitamin B12 solution has been used to coat certain porous polymer materials. Basic fibroblast growth factor (bFGF) was complexed with sucralfate and polyHEMA and then coated on PLLA scaffolds to enhance liver tissue formation.
The above-described methods of bioactive agent incorporation may, in some instances, achieve certain slow release purposes, but the control over release profile of a bioactive agent is generally limited. Further, these methods are not suitable for releasing multiple agents with individualized controlled release profiles.
Attempts have been made to use micro- and/or nano-spheres or particles (also referred to herein as “MNS” and/or “MNP”, these abbreviations meaning herein, micro-spheres/-particles, nano-spheres/-particles, or combinations thereof) made of polymers (often biodegradable polymers) for incorporating drugs and proteins for controlled release. Attempts are also being made to incorporate the controlled release capacity into scaffolding materials to provide desired bioactive agents. However, the successful realization of such ideas to date has proven technically challenging.
One way of combining factor-containing MNS/MNP with scaffolding materials is to directly load MNS/MNP into a scaffold. For example, MNS/MNP containing transforming growth factors (TGF-β1) have been added into a scaffold to aid chondrogenesis. Gelatin MNS/MNP containing bFGF have been mixed with preadipocytes and dripped on collagen sponge sheets to engineer adipose tissue. However, this method is limited because the added MNS/MSP may migrate under gravity, physical motion, mechanical interference (stirring, shaking, mechanical stress and so forth), and/or fluid flow. Such migrations may deleteriously lead to undesired distribution or loss of MNS/MNP.
PLGA MNS/MNP encapsulating IGF-I (insulin-like growth factor) and TGF-β1 have been entrapped together with condrocytes in a photopolymerized PEO (polyethylene oxide) hydrogel for cartilage formation. Advantages of such in situ gelling systems include the ability to fill irregularly shaped tissue defects, allowing minimally invasive procedures (such as injection and arthroscopic surgery), and the ease of cell and MNS/MNP incorporation. However, the disadvantages include the non-degradability of PEO, the lack of control over macro- and micro-pore shape and size in the gels, and poor mechanical properties.
Similarly, bFGF-containing PLGA MNS/MNP have been incorporated into alginate gel and crosslinked. The alginate gel was then lyophilized to form solid porous materials as a membrane to release bFGF for enhanced angiogenesis and wound healing. The advantage of this method is that the solid-state form may be more mechanically stable than gels. The disadvantages include the lack of matrix degradability (alginate), the inability of controlling pore shape, and the limited ability of varying pore size. Although different from hydrogels, calcium-phosphate cement was also loaded with PLGA MNS/MNP to release rhBMP-2 (recombinant human bone morphogenetic protein-2). Unfortunately, this system generally has similar disadvantages such as the lack of degradability, and the inability of controlling pore shape and pore size.
Another method of entrapping PLGA MNS/MNP into a porous scaffold is gas foaming. In gas foaming, carbon dioxide (CO2) is usually used as a foaming agent. Solid polymer disks are exposed to high pressure CO2 to allow saturation of CO2 in the polymer. Thermodynamic instability is then created by rapidly releasing CO2 gas from the polymer system, followed by the nucleation and growth of gas bubbles in the material. This method allows protein or DNA powders or PLGA MNS/MNP to be entrapped in the materials. However, the disadvantages of this method include the lack of control over pore size and shape, and a nonporous surface, closed-pore structure, with only between about 10% and about 30% of interconnected pores. The porosity and interpore connectivity may, in some instances, be improved by combining salt-leaching techniques with the gas foaming process, although eliminating closed pores has yet to be successfully achieved. Additionally, the leaching process may remove a large portion of the incorporated agents in an uncontrolled manner, which is also not desirable. Furthermore, the embedded MNS/MNP in the walls of the porous materials generally lose the desired release profiles of the MNS/MNP.
To make it feasible to suspend PLGA micro-/nano-spheres or particles in a PLGA solution, PLGA MNS/MNP have been coated with a layer of PVA (polyvinyl alcohol). The PLGA solution with suspended PVA-coated MNS/MNP was used with salt particles to fabricate scaffolds based on salt-leaching techniques. Theoretically, the PVA coating does not dissolve in organic solvents such as THF (tetrahydrofuran), thereby potentially preventing the dissolution of PLGA MNS/MNP in a PLGA solution. However, a defect-free PVA layer on a MNS/MNP in suspension has yet to be easily, reproducibly and successfully achieved. Additionally, the embedded MNS/MNP inside the pore walls may not result in controlled release profiles.
As such, it would be desirable to provide controlled release therapeutic materials having desirable control over release profiles of bioactive agent(s). Further, it would be desirable to provide such materials suitable for releasing multiple agents. Still further, it would be desirable to provide such materials with individualized controlled release profiles.